In the majority of oil refining processes employed today, coke is produced from petroleum residual fractions. Environmental regulations throughout the world require that entrained coke particles and volatile components in effluent streams be captured and disposed of to prevent releasing these materials into the environment. Frequently, such effluent is disposed of by burning the mixture in an incinerator.
Steam cracking is the major commercial process for the production of light olefins, especially ethylene and propylene. Light olefins are produced by the thermal cracking of hydrocarbon feeds at high temperature and low hydrocarbon partial pressure for short residence times.
In steam cracking, the hydrocarbon feed is first preheated and mixed with dilution steam in the convection section of the furnace. The temperature exiting the convection section is generally designed to be just at the point where significant thermal cracking begins. After preheating in the convection section, the vapor feed/dilution steam mixture is rapidly heated in the radiant section to achieve the desired thermal cracking. After the desired degree of thermal cracking has been achieved in the radiant section, the furnace effluent is rapidly quenched in either an indirect heat exchanger or by the direct injection of a quench oil stream.
An undesirable byproduct of the cracking process is often the deposition of carbon deposits, commonly referred to as “coke,” on the inner surfaces of the radiant tubes of the furnace. Depending on the feedstock being cracked, coke may also be deposited in certain tubes in the convection section, or in the quench system of the furnace. Where feedstocks containing non-volatile hydrocarbons, commonly referred to as asphaltenes, resid or pitch, are processed in a furnace, including but not limited to those processes in which the convection section is equipped with an intermediate vapor-liquid separator, foulant or coke deposition may be expected on the internal surfaces of the separator.
There is a limit to the quantity of coke that can be deposited in a furnace and still permit normal-range furnace operation. Eventually the coke deposits begin to insulate or clog the tubes and must be removed before either the maximum radiant tube metal temperature (TMT) is reached, the maximum radiant coil pressure drop is reached, the maximum convection section pressure drop is reached, the maximum quench system pressure drop is reached, or, in the case where the furnace effluent is quenched in a steam generating quench exchanger, the maximum quench exchanger outlet temperature is reached.
The effluent from steam-air decoking comprises steam, air, CO, CO2 and uncombusted coke particles. Historically, the effluent from steam-air decoking was directed to a decoke cyclone or decoke drum, where the coke particles were removed and the vapor products rejected to the atmosphere via a decoke vent stack. Depending on the design of the decoke drum or cyclone, a water-wash stream may be used to prevent coke particles accumulating on the walls of the drum. Coke particles are collected from the bottoms of the decoke drum and may be disposed of by land-fill, as a by-product, or incineration.
More recently, furnace designs have become available that direct the effluent from the steam-air decoke back to the firebox of the furnace rather than to a decoke drum. In this way the CO in the decoke effluent steam is converted to CO2, and the intent is that any unburned coke particles will be incinerated.
In such designs, a decoke effluent stream is injected through one or more nozzles in the floor of the furnace to maximize the residence time for incineration of coke particles in the radiant firebox. A typical arrangement for the injection of decoke effluent into a single-tube-plane firebox is illustrated in FIG. 1.
As illustrated in FIG. 1, steam-air-decoke effluent is injected vertically into the radiant firebox through nozzles 4 mounted in the floor of the furnace. On one side of the injected decoke effluent stream is the flame from furnace floor mounted burners 5. On the other side of the decoke effluent stream is the plane of radiant tubes 3 being decoked. In such designs, the decoke effluent is injected into the radiant firebox 2 with a flame on one side and the cooler radiant tubes 3 on the other side.
In furnace designs of the type depicted in FIG. 1, there exists the potential for erosion of the radiant wall refractory by the abrasive coke particles as the turbulent decoke effluent stream expands after leaving the injection nozzles. Moreover, it can be undesirable to inject the decoke effluent stream in close proximity to the radiant tubes of the furnace, as the tubes are the coolest surface existing in the radiant firebox and may retard the combustion of the coke particles.
Therefore, what is needed is an improved furnace configuration that enables the injection of decoke effluent into the firebox without the deficiencies of prior designs.